In ORAS, there's this handy little features called the Dex Nav. It allows you to search for Hidden Pokemon (Pokemon that appear in the Dex Nav), which can get you encounters without having to search in the tall grass. It also allows you to chain Pokemon for Higher Levels, Hidden Abilities, Perfect IVs, Egg Moves, items (for certain Pokemon), and gets you a higher shiny chance (this is a heavily oversimplified explanation, you can read more about it here.)
Now, after you obtain the National Dex (which is Gens 1-6 for these games, you get it after beating/catching the cover legendaries in ORAS), the range of Hidden Pokemon increase and allows you to search for specific Pokemon that aren't in the Regional Dex, depending on the Route you're on. Examples include Sewaddle, Zorua, and Lillipup on Route 101, Skorupi, Misdreavus, and Tympole on Route 114, and Trubbish, Chatot, and Shellos on Route 110. Shellos is only obtainable in ORAS using the Dex Nav after you get the National Dex, since it doesn't appear in the Regional Dex.
Additionally, if you clicked on the Route links, you'll see the encounters I listened under the Dex Nav section, which is specifically for Pokemon who appear via the Dex Nav after you get the National Dex. I can understand your confusion though, since the question you linked has no direct mention of the Dex Nav.
In short, the Database isn't wrong, you just needed to have knowledge of the Dex Nav. I hoped this cleared up any confusion.
